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Old 21st May 2004, 03:26 PM   #1
troystg is offline troystg  United States
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Default parallel voltage regulators to increase current?

Hi All-

I was curious about what would happen if you paralleled voltage regulators to try and get more current availability for a circuit.

I did a search and did not find any drawings such as below or this specific question being "recently" discussed. I was not about to read search results going back 50 yrs (exaggerating of course).

Thanks in advance,

Troy
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Old 21st May 2004, 03:28 PM   #2
troystg is offline troystg  United States
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By the way, I drew the picture with Visio in 5 minutes flat so please excuse the lack of detail and pin assignments.

Troy
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Old 21st May 2004, 03:33 PM   #3
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I most probably won't work. Since two devices are never
exactly identical they will fight each other and share the
current unequally. I would even be worried about oscillations
due to the mutual independent feedback loops.

There has been suggestions before on the forum to parallell
regulators by adding series resistors at their outputs. That
might work, but regulation will suffer.
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Old 21st May 2004, 03:37 PM   #4
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I agree with Christer. I think it might work with 317-type (or other with external feedback network) regulators, by including a small sub-ohm (depending on current drawn) equalising resistor and taking the feedback point for all regulators at the load point where the equalising resistors come together.

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Old 21st May 2004, 03:40 PM   #5
joensd is offline joensd  Germany
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You might wanna look into the datasheets of the IC´s as there is extensive information about these applications.

As Christer said your circuit won´t work very reliable but some chips from Linear Technology for example (LT1083) just need very low ballast resistances on the output (like 0.015Ohm) to share current equally.
They are more expensive of course.

You´ll also find application notes from National Semiconductor, Linear Technology about voltage regulators which are very interesting reads.

You also get voltage regulators up to 10A but it depends on your application if you can get away with it.
Maybe you´re doing it to spread power dissipation between the IC´s?
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Old 21st May 2004, 03:48 PM   #6
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Hi Troy.

In the LMx17-datasheet, You can find this:
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Old 21st May 2004, 04:01 PM   #7
troystg is offline troystg  United States
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Hi All-

Well I'm building a pwr supply box for a friend who has six(6!) 9 volt wall warts powering up his guitar pedals.

I got a transformer, bridge, plenty of filter caps and 4 7812 regulators. Instead of having 4 separate circuits for each regulator(2 pedals per each 1 amp regulator) I was looking into just paralleling the 4 7812's to see if I could get them to split the 4 amp load.

I guess it would be safer to have the separate circuits for each regulator.

Not critical circuits or anything. By the way, all of the pedals have an operational range of 9-15 volts DC.

Thanks again,

Troy
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Old 21st May 2004, 04:03 PM   #8
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One of the best and most popular ways to achieve greater current output of a regulator, is to use parallel emitter current followers on the output.

I don't have any schematic drawing software handy, however, there are many good examples on the internet, and probably many users here at diyaudio that could provide a simple example.

Essentially, the output of a positive regulator will connect to the base of an NPN transistor, with the unregulated DC voltage input also going to the collector of the transistor, and the emitter being the new output. It works on the principle of maintaining vbe drop across the transistor. The ouptput will now be about .5 to .8 volts lower than the original regulator output, so it's nice if you have a variable regulator like a 317 so you can set the desired output.

This may be a bit confusing, but if someone shows a diagram, it'll all clear up.
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Old 21st May 2004, 04:09 PM   #9
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Is good regulation critical? I get a feeling that is not the case
in your application. If it isn't then maybe it is better to just
build a simple unregulated supply based on a zener and an
emitter follower?
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Old 21st May 2004, 04:19 PM   #10
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Have you looked at the specifications of the pedals?
Usually they use currents much below 100mA each.
I used them myself but the only one that´s left is a tube pedal consuming up to 400mA but that´s the tip of the iceberg.
With 200mA per pedal that would sum up to 1.2A.
I´d use just one 1.5A, 2A, or 3A regulator (LM317,L200, LM350.....) depending on your needs.
Important is that the power supply is absolutely quiet so proper bypassing and enough capacitance is required.

If you have the 7812 around and really need more than 1A you could also use one 7812 for 1 (or2) pedals.
This way you don´t need to worry about current sharing the whole load.
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